A variety of electrical devices use dielectric materials of various properties for various purposes. For example, materials with moderately high dielectric constants are used in such devices as dielectric resonator filters, microwave stripline circuits, various types of oscillators, as well as phase shifters, to name but a few. Dielectric constant is an important variable in the design of such devices, but equally important are low loss and temperature stability. For one class of devices, low loss is necessary to prevent dissipation of the electrical signal and for the design of circuits with high Q and narrow bandwidth. Temperature stability is required to prevent frequency changes in these devices. Good temperature stability permits much closer control of frequency characteristics when external temperature stabilization is used and may eliminate need for such stabilization in some applications. In addition, external temperature stabilization may not correct for temperature changes due to microwave heating of the dielectric material.
The temperature coefficient of interest here is the one determined by changes of resonant frequency of a dielectric resonator. This effective temperature coefficient includes thermal expansion effects as well as dielectric effects. The effective temperature coefficient is defined by the equation: ##EQU1## in which f is the resonant frequency. It should be noted that .tau..sub.eff is also often used to characterize dielectric material in this field. The quantity .tau..sub.eff and (TCF) are related by the equation .tau..sub.eff =-2(TCF).
The initial widespread use of dielectric material in microwave devices occurred with the discovery that Ba.sub.2 Ti.sub.9 O.sub.20 had unusually low temperature coefficients together with high dielectric constants and low microwave losses (high Q). This material is described in a number of references including U.S. Pat. No. 3,938,064, issued to H. M. O'Bryan, Jr. et al on Feb. 10, 1976 and U.S. Pat. No. 4,337,446, issued to H. M. O'Bryan, Jr. et al on June 29, 1982.
The materials disclosed in the references cited above usually had temperature coefficients of resonant frequency (TCF) of about 2-3 ppm/.degree.C. This indeed made it possible to use this material in many applications.
In other applications, still lower (TCF) values were desirable and even negative values of (TCF). This was especially desirable where dielectric heating effects were large or where the device structure made a large contribution to the temperature coefficient. In this latter situation, a negative (TCF) would be desirable to compensate for the contribution to frequency drift due to the device structure.
Also desirable from a fabrication point of view is a procedure for adjusting the (TCF) value for different microwave devices.